This invention includes a control device and a maintenance support device. The maintenance support device included in the plant monitor and control device selects temporary setting target logic element among logic elements contained in a control logic data, generates a temporary setting target logic element data, and generates a temporary setting logic element parameter data in accordance with a value of logic element parameter inputted by an operator. The control device memorizes the temporary setting target logic element data sent from the maintenance support device as the temporary setting target logic element data, and also the logic element parameter data before the temporary setting corresponding to the logic element designated by the temporary setting target logic element data among the logic element parameter data. Then, the control device renews the logic element parameter data located in the saving area with use of the temporary setting logic element parameter data sent from the maintenance support device.